text: Simply due to the high number of microorganisms in membrane bioreactors, pollutant uptake rates can be increased. This leads to better degradation in a given time span or to smaller required reactor volumes. In comparison to conventional activated sludge process treatments which typically achieve 95 percent removal, removal can be increased to 96 to 99 percent in membrane bioreactors (see table,). Chemical oxygen demand (COD) and biological oxygen demand (BOD5) removal is found to increase with mixed liquor suspended solids concentration. Above 15 g/L, COD removal becomes almost independent of biomass concentration at >96 percent. Arbitrary high suspended solids concentrations are not employed, however, lest oxygen transfer be impeded due to higher viscosity and non-Newtonian viscosity effects. Kinetics may also differ due to easier substrate access. In typical activated sludge process treatment, flocs may reach several 100 μm in size. This means that the substrate can reach the active sites only by diffusion which causes an additional resistance and limits the overall reaction rate (diffusion-controlled). Hydrodynamic stress in membrane bioreactors reduces floc size (to 3.5 μm in side stream configurations) and thereby increases the effective reaction rate. Like in the conventional activated sludge process, sludge yield is decreased at higher solids retention times or biomass concentrations. Little or no sludge is produced at sludge loading rates of 0.01 kgCOD/(kgMLSS d). Because of the imposed biomass concentration limit, such low loading rates would result in enormous tank sizes or long hydrodynamic residence times in conventional activated sludge processes.